The present invention generally relates to a gradation correcting apparatus which is capable of correcting the gradation of picture signals in picture apparatuses such as television image receiving machines, video tape recorders and so on.
In recent years, the gradation correcting apparatus is regarded as important in terms of the more dynamic expression of the picture signals on a cathode-ray tube (CRT) as the television image receiving machines become larger in size, and higher in picture quality.
The conventional gradation correcting apparatus will be described hereinafter in one example with reference to the drawings.
FIG. 3 is a block diagram of the conventional gradation correcting apparatus. In FIG. 3, reference numeral 1 is an analog to digital (A/D) converter for converting an input luminance signal a, which is an analog signal, into a digital data so as to output a luminance signal data b. Reference numeral 2 is a histogram memory. The histogram memory is a memory where a memory address is set corresponding to the luminance signal data b, the data of the address data shows a histogram corresponding to the respective input luminance the data of the luminance is read as a histogram memory output data e in accordance with the input luminance. In the histogram memory 2, after a particular summing data has been calculated into a histogram memory output data e by an externally provided adder 9, it is stored in an address corresponding to the luminance of the input luminance signal (histogram memory input data f). Here the particular summing data means, for example, "1". Reference numeral 4 is a cumulative addition normalized arithmetic unit, which cumulatively adds as the input the histogram data c from the histogram memory 2, normalizes so that the maximum value after the cumulative addition thereof may become a maximum value of the output luminance so as to output the cumulative normalized histogram data d. Reference numeral 5 is a look up table which has memories therein. The look up table 5 has addresses established in accordance with the luminance of the luminance data b, stores the cumulative .multidot. normalized histogram data d in the address, reads the data of the address corresponding to the luminance data so as to be synchronized with the input of the luminance data b, and outputs it as an output luminance signal data g. Reference numeral 6 is a digital to analog (D/A) converter, which converts the output luminance signal data g into the output luminance signal h of the analog so as to output it.
The gradation correcting apparatus constructed as described hereinabove will be described hereinafter in its operation with reference to FIG. 3 and FIG. 4.
The input luminance signal a is converted into the luminance signal data b by the AD converter 1. The data of the address of the histogram memory corresponding to the luminance signal data b (luminance thereof) is selected and becomes the histogram memory output data e. The data e has the addition data (1) added by the adder 9, becomes a histogram memory input data f, is stored again in the previous address of the histogram memory 2. The histogram is provided about one sheet of or several sheets of pictures in this manner. The histogram obtained in this manner is shown in, for example, FIG. 4 (a) After the histogram has been provided, the data (histogram data c) from the histogram memory 2 is read into the cumulative addition .multidot. normalized arithmetic unit 4 in order from the smaller address (smaller input luminance) The cumulative histogram is calculated in the arithmetic unit 4, and the maximum value of the cumulative value is normalized so that it becomes the maximum value of the input luminance signal data. This is shown in FIG. 4 (b) The data whose normalization has been finished is inputted into the look up table 5 as the cumulative .multidot. normalized histogram data d. The look up table 5 is composed of random .multidot. access .multidot. memory (RAM), where the address is used to become the input luminance, the data of the address is used to become the cumulative normalized histogram data d. When the setting of the data d is finished in the look up table 5, the gradation correction is effected. In the gradation correction, the input luminance signal data b is inputted into the look up Table 5, an address is set in accordance with the data b to read the data of the address. This becomes an output luminance signal data g (FIG. 4 (c)). The data g is inputted into the D/A converter 6, and is converted into an analog amount. The output luminance signal h is obtained, which allows the gradation correction to be effected. The above described processing is generally called histogram conversion processing in the field of the picture processing.
In such a conventional gradation correcting apparatus, no problems are caused in the processing of black and white signals. When the processing has been applied to the color picture signals, problems are provided in that the portion where the gradation wants to be actually enlarged, the portion near the I shaft of NTSC system regarded as colors sensitive to, for example, the human faces, eyes are not sufficiently enlarged in gradation, and so on, because the color information is not inputted.